This invention relates to a control method for controlling the fuel supply to an internal combustion engine at deceleration, and more particularly to such a method in which the quantity of fuel being supplied to the engine is reduced in a manner adapted to the actual engine operating condition while the engine is decelerating, to thereby prevent the air/fuel mixture being supplied to the engine from becoming over-rich.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Pat. No. 3,483,851, which is adapted to determine the valve opening period of a fuel quantity metering or adjusting means for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to this proposed control system, if the setting of the fuel supply quantity is made on the basis of such basic value as a function of the engine rpm and the absolute pressure in the intake passage of the engine, in the above explained manner, independently of a sudden reduction in the supply of suction air to the engine due to the closing of the throttle valve at engine deceleration, there can occur an excessive supply of fuel to the engine due to a time lag in the amount of drop in the absolute pressure in the intake passage of the engine corresponding to changes in the throttle valve opening. That is, when the throttle valve is abruptly closed, the drop in the absolute pressure in the intake passage can not at once follow such a change in the throttle valve opening, and the absolute pressure in the intake passage continues to drop even after the throttle valve has completely been closed. Also, there can occur a delay in the detection of the actual absolute pressure in the intake passage due to a time lag occurring in the absolute pressure detecting sensor means to respond to the actual absolute pressure in the intake passage.
If the fuel supply reduction quantity at engine deceleration is set in response to changes in the throttle valve opening, as explained hereabove, such reduction in the fuel supply to the engine is terminated before the absolute pressure in the intake passage drops to a sufficiently low level, resulting in the air/fuel mixture being supplied to the engine becoming over-rich due to discontinuation of the fuel supply reduction after the full closing of the throttle valve, thereby badly affecting the emission characteristics and fuel consumption of the engine.